Electrical components consist of a plurality of structured layers, which are created individually and in succession until the component is completed. Each layer is transferred onto a semiconductor substrate, the so-called wafer, by photolithography, with a so-called mask serving as a template in each case. In general, a mask comprises a transparent carrier material, for example quartz glass, and a non-transparent material, generally referred to as an absorber. This absorber is structured such that it produces bright and dark regions on the wafer when the mask is imaged. A photosensitive layer on the wafer, the so-called resist, reacts with the incident light, as a result of which the resist is structured in accordance with the mask template. Hence, the desired structures ultimately arise on the wafer.
A widespread problem when structuring the absorber on the mask consists of producing the structures exactly according to the dimensional specifications. As a rule, real masks have a certain variance, for example in a line width which, according to the prescriptions, should be constant. Here, the so-called CDU (critical dimension uniformity) is a measure for the line width variance. This measure decisively determines the quality of the mask. If the CDU of a mask exceeds a certain measure, the mask counts as non-usable and hence as a reject because the uniformity of the line widths on the wafer plays a decisive role for a high yield of functioning electrical components. What makes matters even more difficult here is that, as a result of the lithographic method in the scanner, the line width variance produced on the wafer by the mask is higher than the line variance of the absorber on the mask by a factor, the so-called mask error enhancement factor (MEEF).
Although etching methods can be used to modify the structures on the mask to a certain extent within spatially tightly delimited regions, the problem of a larger-area modification regularly arises. Therefore, according to the prior art, the so-called CDC (critical dimension control) tool is used for such modifications. Local scattering centers, so-called pixels, or whole regions with pixels, i.e., changes in the material structure of the mask, are written by use of the CDC tool using a femtosecond laser. Usually, regions provided with pixels with diameters in the region of several millimeters or centimeters should be created. Since the pixels are written into the quartz glass and consequently found in the optical path upstream of the absorber, the incident light is scattered at these pixels during the exposure process, as a result of which some of the light no longer reaches the absorber of the mask. Consequently, the intensity of the light reaching the absorber is influenced by varying the pixel density. The intensity changes triggered hereby in turn cause a line width change on the wafer during the exposure process. Should the intensity with which the absorber is exposed now be modulated using this technique in accordance with the known CDU of the mask, it is possible to compensate the line variance of the mask for the imaging in the scanner. Expressed differently, by writing the pixels, the line width that physically deviates on the mask is corrected on the image that is created on the wafer. However, the absorber on the mask is not physically modified in the process; instead, all that changes is the imaging thereof when exposing the wafer in the scanner.
Usually, the CDU is measured during the production process of the mask. This is brought about, inter alia, with the aid of a mask metrology device, i.e., an optical apparatus that emulates the most important optical properties of a scanner and therefore inherently captures some of the effects contributing to the aforementioned MEEF. Examples of these apparatuses include the wafer level critical dimension measuring appliance, abbreviated WLCD, and the aerial image measurement system, abbreviated AIMS™, with the first being used in dedicated fashion for measuring the CDU.